This invention relates generally to steam turbines and, more particularly, to methods and systems for fabricating a rotor for a steam turbine.
At least some known rotors are fabricated as a single forging that includes rotor ends, bearing regions, packing regions, and a steampath section. Generally, the weight of such rotors causes the rotor to pass through a first critical speed during operation. Specifically, the first critical speed is equal to the square root of the rotor's stiffness over the rotor's weight. More specifically, the first critical speed may be mathematically represented as:
            critical      ⁢                          ⁢      speed        =                            k          2                w              ,
wherein k represents the stiffness of the rotor and w represents the weight of the rotor. As such, an increase in the weight of the rotor results in a lower critical speed. At critical speed, because the rotor rotates at a frequency that is generally equal to the natural frequency of the rotor, rotor vibration may become unstable. To avoid damage to the rotor and/or engine, the rotor must either be operated at a speed that is less than the first critical speed, or the rotor must be quickly accelerated to an operating speed that is faster than the first critical speed.
Other known rotors are designed to have less weight, such that the first critical speed is increased. At least some of such rotors include a bore that extends substantially concentrically through the rotor shaft. However, to satisfy structural requirements such known rotors are generally fabricated with a large wall thickness as measured between the outer bore diameter and the outer rotor diameter. As such, the solidity of such rotors is generally not reduced enough to enable the rotor to be operated at a speed that is less than the first critical speed.